As disclosed in National Publication of International Patent Application No. 10-504397 and others, methods are available in which an analysis object to be tested is disposed in a certain portion on a disc and the analysis object is traced to obtain an image of the analysis object by using the reproducing function of the optical disc.
As shown in FIGS. 7 and 8, an optical disc 101 generally has tracks 102 of an aluminum reflective layer that are formed on a surface of a base 101b. Information is recorded on pits and grooves 103 which are asperities finely formed on the tracks. Reference numeral 104 denotes a protective layer.
In a typical optical disc drive shown in FIG. 6, reading is performed on the tracks 102 by a laser beam Ph from a pickup 106 while the optical disc 101 is rotated in the direction of arrow C by a disc motor 105. The pickup 106 is screwed onto a feed screw 109 driven by a traverse motor 108. A servo control circuit 107 drives the traverse motor 108 to move the pickup 106 in the radial direction in such a way that the tracks 102 are traced according to the reproduction output of the pickup 106. Further, the servo control circuit 107 detects address information recorded on the tracks 102 and drives (CLV control) the disc motor 105 with a constant linear velocity.
To be specific, regarding the irradiation position of the laser beam Ph on the optical disc 101, the optical path of the laser beam Ph is driven not only by driving the traverse motor 108 but also a tracking actuator (not shown), which is provided in the pickup 106, in the lateral direction (radial direction) with respect to a surface of the optical disc 101 as necessary, and the tracks 102 are accurately traced while tracking control is performed.
In the case of an analysis disc unlike audio CDs and video CDs, an analysis object 110 is further disposed in the optical disc 101 as shown in FIGS. 7 and 8. In an analysis device using the technique of a conventional optical disc drive, the analysis object 110 is read by the pickup 106 and is processed by a video signal processing circuit 111 to obtain an image of the analysis object 110.
When using the address information of a coded signal written on the track on the disc, the address information may not be captured well due to a servo disturbance of focusing and tracking during the passage of the analysis object 110.
On a part where the analysis object 110 is present, the pits and grooves cannot be disposed. Even if the pits and grooves are present, it is quite difficult to read signals on the pits and grooves because of the influence of the analysis object 110. Such a part corresponds to a defect on an optical disc and thus a rotary servo, a tracking servo, and a focus servo on the optical disc do not normally operate when passing through the part. As a result, a number of problems frequently occur. For example, the rotation of the disc is disturbed during the passage through the part corresponding to the analysis object 110, and an adjacent track is caught after the passage of the part corresponding to the analysis object 110. Some measures are available to keep each servo in a holding state only on a defect. However, the holding state cannot be positively kept because each servo is out of control. As the number of disposed analysis objects increases, the number of unstable regions increases, resulting in greater influence of the servo disturbance. Further, similar problems occur in the rotary servo control of a disc.